The phases of the moon are caused by the changing orientation of the illuminated half of the moon relative to the Earth as the moon orbits our planet. There are eight phases of the moon: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent. As the moon revolves around Earth, the portion of its surface that is illuminated by the sun varies based on its position.